Traveling wave tube



June 20, 1961 c. c. CUTLER 2,939,661

TRAVELING WAVE TUBE Filed April 26, 1956 F/G. /B

INVENTOR C. C. CUTLEA ATTORNEY 2,989,661 TRAVELING WAVE TUBE Cassius C.Cutler, Gillette, NJ., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed Apr. 26,1956, Ser. No. 580,908 14 Claims. (Cl. 315'--'3.6)

The present invention relates to a novel transmission circuit for use ina traveling wave tube.

Traveling wave tubes of various types are used for amplifying highfrequency wave energy. Such amplification is achieved by propagatingwave energy along a specially designed transmission circuit whileprojecting an electron beam along a path in coupling relation with theelectric field of the propagating wave. Interaction between the electricfield of the wave and the electron beam occurs, and when the velocity ofpropagation of the wave and the velocity of the electrons of the beamare approximately synchronous, the interaction produces an amplificationof the wave. Because the maximum beam velocity obtainable in a practicaltube is substantially less than the velocity of wave propagation alongmost transmission lines, it is necessary to provide a socalled slow wavetransmission circuit for reducing the effective velocity of wavepropagation.

Many such slow wave circuits have been proposed. Perhaps the mostsuccessful and certainly the most commonly used circuit comprises aconductive member, such as a wire or conductive tape, wound in the formof a helix. This circuit is characterized by structural simplicity and acapacity to operate over a wide frequency band. With such a circuit anelectron beam is generally propagated along the axis of the helix and ismaintained substantially synchronous with the axial velocity componentof a wave propagating along the helix. Thus amplification is achieved.

A disadvantage of the helix transmission circuit for many purposes isthat the amount of power obtainable from such a circuit is limited. Thisis particularly true for operation at very high frequencies since thedimensions of a helix suitable for use at such high frequencies areextremely small. Hence only a low current beam can be projected alongthe helix axis without striking the helix and thereby causing distortionto the amplified wave and damage to the fragile helix. Efforts toincrease the power obtainable by arranging several helices in parallelhave not been successful. These efiorts have gone in two directions. Thefirst is merely to arrange a bank of tubes in parallel, each of whichemploys a helix propagating circuit. This technique is not completelysatisfactory because it is unwieldy and, further, because difficultphasing problems are introduced.

The second technique is to braze several identical helices together toform a unitary wave propagating structure of several helices inparallel. The circuit thus formed is then to be incorporated in a singletube. This latter technique has proved unsuccessful principally becauseof three factors. Firstly, it is now appreciated that such a structureis unsuited for a wave propagation circuit because the coupling betweenthe several helices is so high that the several helices of the structureno longer act as independent circuits arranged in parallel, but, rather,as coordinate parts of a single circuit formed by the entire structure.Moreover, the resulting circuit thus formed is characterized by twophase velocities, one slower than and one faster than the phase velocityof the individual helices. Each of these phase velocities differs fromthe phase velocity of the individual helices by an amount dependent uponthe degree of coupling between the helices. In the limiting case, thatis, as the coupling A United States Patent 9 2,989,661 Patented June 20,1961 between the helices approaches unity, they approach zero andinfinite velocity, respectively, and so the circuit is nonpropagating.However, even where the coupling remains low enough to permitpropagation, neither of the two phase velocities is especially suitablefor interaction with a beam and any interaction obtained is of a narrowband character. The second factor militating against successfuloperation in the brazed multiple helix arrangement is thatdiscontinuities are disadvantageously introduced at the points where thehelices were brazed together. Thirdly, the various helices must bedimensioned virtually identically in order to maintain constant phaserelations between the electrical fields of the waves passing along theseveral helices, but such identical dimensioning is difficult to achievein helix structures heretofore proposed.

It is a principal object of the present invention, therefore, to achievesatisfactory high power operation from a helix-like transmissioncircuit. A more particular object is to achieve such operation in ahelix-like transmission circuit which may be readily fabricated.

To this end, a principal feature of the present invention is a wavepropagating circuit for use in a traveling wave tube which essentiallycomprises a plurality of parallel extending helices, the pitches ofadjacent helices of which are opposite in sense. The coupling betweensuch oppositely pitched parallel extending helices can theoretically bezero. Thus each of the various helices acts as an independent circuithaving the same phase velocity it would have in the absence of the otherhelices. In this way eflicient interaction with the beam can beachieved.

A second feature of the present invention is a wave propagatingstructure of the above type formed by a plurality of wire-likeconductive elements arranged in a matrix-like structure. The matrixstructure in a preferred arrangement comprises a longitudinal successionof sets of straight vertical elements and straight horizontal elementsarranged in a manner to simulate in effect a plurality of identicalhelical conductive paths longitudinally through the array. Actually,there is formed a plurality of identical conductive paths which are nottruly helical only in that their shape in a projected end view isrectangular rather than circular. Moreover, the pitches of adjacenthelical paths through the array are opposite in sense. The use of onlystraight elements considerably facilitates the ease of manufacture ofthe structure.

In an illustrative embodiment, the invention comprises a traveling wavetube including an electron gun and a target electrode spaced apartwithin an evacuated envelope for defining a path of electron flow withinthe envelope. A slow wave transmission circuit is located in couplingrelation with the path of flow and extends along a substantial portionthereof. This circuit comprises a longitudinal succession of sets ofvertical elements interleaved with a longitudinal succession of sets ofhorizontal elements in a manner to be described in detail hereinafter toachieve the efiect of a plurality of parallel helical paths.

The invention will be explained in greater detail in the followingdescription taken in conjunction with the accompanying drawing, inwhich:

FIG. 1A is a perspective view of a traveling wave tube, shown partiallycut away for purposes of illustration, which includes a slow Wavetransmission circuit designed in accordance with the present invention;

FIG. 1B is a cross-sectional view taken looking to the right along plane1B--1B of FIG. 1A;

FIG. 2A is an expanded view of a fragment of the transmission circuitincluded in the tube of FIGS. 1A and 1B; and

FIG. 2B is a plan view of the transmission circuit fragment of FIG. 2A.

Referring now more particularly to the drawing, FIG. 1A shows atraveling wave tube comprising an evacu-. ated envelope 11, typically ofglass or a nonmagnetic metal such as copper, enclosing an electron gun12 for forming an electron beam and projecting the beam along anextended longitudinal path toward collector 13. The electron gun, asshown schematically, includes a cathode 15, heater 16, beam formingelectrode 17, and accelerating anode 18. In practice, these elements aremaintained in place by suitable supporting members, and there areprovided lead-in conductors from suitable voltage sources formaintaining the various elements at appropriate potentials. Inparticular, the cathode customarily is biased to a potential slightlypositive with respect to the beam forming electrode and appreciablynegative with respect to the accelerating anode so that a beam ofelectrons will be projected toward the collector which is also biasedpositively with respect to the cathode. Additionally, in practice,apparatus is generally provided for establishing a magnetic field formaintaining the beam focused along this path. This apparatus may, forexample, include a solenoid surrounding the beam along its length or apair of oppositely poled permanent magnets spaced at opposite ends oftube 10 for providing a longitudinal magnetic field.

A transmission circuit 22, comprising a matrix-like array of straighthorizontal and vertical wire elements shown in phantom in FIG. 1A, ispositioned along a major portion of the beam path, such that each of thewave paths through the circuit is a plurality of operating wavelengthslong. The details of the circuit will be discussed hereinafter. Thecircuit is maintained at a suitable positive D.-C. potential withrespect to the cathode. Such potential fixes the velocity of theelectrons in their flow past the circuit and is adjusted to achieveamplification of the wave traveling along the circuit.

The circuit is advantageously supported along its length by envelope 11,as can be seen from FIG. 1B. As indicated above, envelope 11 may beeither glass or a nonmagnetic conductor such as copper. In either eventsuitable connections can be made. When using glass, the wire-likeelements of the circuit can be embedded in the glass. When a copperenvelope is used, the elements can be brazed to the copper. A techniquewhich has been found to be satisfactory when using a copper envelope isto form the envelope by use of a succession of washerlike elements. Inthe process, two wire-like elements were placed across the aperture ineach washer and brazed at their ends to the washer. The washers werethen packed together to form an envelope containing circuit 22, whichwill be viewed in greater detail in FIGS. 2A and 213. An alternativetechnique for supporting the circuit, either in a glass or conductiveenvelope, is to adjust its dimensions to achieve a snug fit within theenvelope and then to secure some part of it fixedly to the envelope toprevent rotation.

Where the circuit makes contact with the envelope for support, the useof a conductive material for the envelope will affect thecharacteristics of the circuit. In particular, with a conductiveenvelope the bandwidth of the circuit will be reduced since a lowfrequency cut-off will occur at a point where dimension 1 of FIG. 1B isapproximately a quarter wavelength. Such a reduction in bandwidth,however, is accompanied by an increase in circuit interaction impedance,and hence in circuit gain, for operation near the cut-off frequency.Thus where extremely broad band operation is essential, a glass envelopeis advantageously used. But where less stringent bandwidth requirementsobtain, higher gain can be achieved by using a conductive envelope andoperating near the cut-off frequency.

An enlarged view of a fragment of circuit 22 appears in FIGS. 2A and 2B.The details of the circuit can be seen more clearly in these figures.The fragment shown comprises in turn with distance in the direction ofbeam flow: a pair of wire-like elements 101a, 101C in a first layerextending in a direction substantially vertical in the drawing; a secondpair of elements 102e, 102g in a second layer extending in a directionsubstantially horizontal; a third pair 103b, 103d extending verticallyin a third layer and interposed in a transverse direction with elements101a and 101c; a fourth pair 104), 10412 extending horizontally in afourth layer and interposed in a transverse direction with elements 102aand 102g. The elements of the fifth layer are aligned with elements ofthe first and the pattern of the first four layers is repeated. Inpractice the circuit typically will include approximately 40 layers, butmay include more or less if more or less gain is desired.

The circuit accordingly comprises in a broadly descriptive sense apattern of sets of four layers iterative in the direction of electronflow. Each layer comprises a plurality of spaced parallel straightwire-like elements lying in a plane transverse to the direction of beamflow. Elements in alternate layers are parallel to one another whileelements in contiguous layers are perpendicular to one another. In adirection transverse to the path of electron fiow, elements of the firstlayer are interposed between elements of the third layer and elements ofthe second layer are interposed between elements of the fourth layer.

The structure thus formed results in a plurality of conductive pathsthrough circuit 22 each of which is essentially helical except thattheir shape in a projected end view would form a square rather than acircle. One of these paths is shown in FIG. 2A by dashed line 23 whichstarts along the first element 1010, then continues right along element102e, down along element 103d, left along element 104 up along thesucceeding element 101a, and so forth through the array. The sense ofthe pitch of the helical-like path thus formed is clockwise in thefigure. An adjacent path, wound in a counterclockwise sense, is shown bydashed line 27. Seven other similarly helical-like paths can be tracedthrough the circuit, adjacent ones of which are wound in opposite sense.In the figure, the shape of each of the helical-like conductive paths ina projected end view would form a square. Alternatively, these shapesmay be rectangular, for example, where the spacing between the variousvertical elements is difierent from the spacing between the varioushorizontal elements, or parallelograms where the elements of successivelayers are not horizontal and vertical but cross at an angle other thanAdditionally, by appropriate modifications the shapes may take the formof any regular geometric figure having an even number of sides, forexample a hexagon, and yet maintain the helix-like paths through thearray.

A wave to be amplified is transferred to each of the roughly helicalpaths by coupling in-phase to elements 101a, 1010 of the first layer.This can be seen more clearly by referring back to FIG. 1A where coaxialline 24 serves to transfer a signal wave in-phase to the two wire-likeelements at the upstream end of the circuit. The signal circuit currentof the wave thus supplied is divided into several portions, each ofwhich passes along a differ ent one of the helical-like paths throughthe circuit. Each of the components is amplified in passing from left toright along the circuit and the several components are recombined at thetwo wire-like elements furthest downstream along the circuit. Theamplified wave is then transferred to an external circuit (not shown)via coaxial line 25, whose center conductor is connected to the twoelements furthest downstream. The amplification is achieved byinteraction between the various signal wave components and the electronbeam whose outer periphery is shown by dashed lines 26 of FIGS. 1A and1B. Some electrons of this beam will strike the wire-like elements atthe upstream end of circuit 22 and the remainder of the electrons willcontinue along the longitudinal passages through the circuit. Thecircuit may readily be made sufficiently rugged to withstand theelectrons impinging thereon, particularly Where the circuit is supportedwithin, and has good heat conduction paths to, a conductive envelope.Nevertheless, the electron impingement can be minimized by inserting aconductive intercepting grid between the electron gun and the circuit.The grid will advantageously have substantially the same configurationas circuit 22 viewed from one end, as in FIG. 1B. Hence it will serve tointercept the electrons which would in its absence strike the circuit.

I In circuit 22 there is no problem of introducing an undesired phaseshift between the various components of the wave energy whose circuitcurrents pass along the different helical paths through the circuit.This is so because the various helical paths of the circuit can readilybe made identical. Additionally, these helical paths are excitedsymmetrically by the coaxial line input connection, and further thevarious helices are interconnected at various points along theirrespective lengths. This interconnection assists in maintaining constantphase conditions along the several helices.

It is to be noted that the pitches of adjacent helical paths through thematrix-like array 22 are opposite in sense. It is this characteristic,as discussed above, which ensures the absence of undesired couplingbetween adjacent helices and thus makes possib e wave propagation by thecircuit at a phase velocity suitable for interaction with the beam.Moreover, without the undesired coupling, each of the several helicesserves as a substantially independent propagating circuit having a phasevelocity substantially the same as it would be in the absence of theother helices. Thus if different signals were applied to the differenthelical paths through the matrix-like array, each of the signals wouldbe amplified. In this case, separate input and output connections wouldbe provided to each of the different helical paths and a separateelectron beam projected axially through each such path. In such anarrangement, the array serves as a multi-channel device. Forfacilitating separate excitation of the different helical paths in amulti-channel device, it will be desirable in some cases to separate thehelices in space while maintaining the pitch of adjacent ones oppositein sense.

It is understood that the embodiment described is merely an illustrativeexample of the present invention. Other embodiments can be made by thoseskilled in the art in the :light of this disclosure without departingfrom the spirit and scope of this invention. Additionally, modificationsof the illustrative embodiment can likewise be made. In particular,circuit 22 need not be supported by the tube envelope; other techniquesknown in the art can be used, such as supporting rods of dielectricmaterial extending longitudinally along the circuit parallel to theelectron beam axis. Further, the circuit has been described ascomprising a succession of layers each of which includes two wire-likeelements. In an alternative arrangement each layer may include a greaternumber of wire-like elements. Additionally, adjacent layers of wirelikeelements have been described as being perpendicular to each other but,if desired, they may be disposed at some other angle.

What is claimed is:

1. In a device which utilizes the interaction between an electromagneticwave and an electron beam for amplifying the wave, a slow wave circuitfor propagatingan electromagnetic wave in coupling relation with saidbeam comprising a plurality of wire-like conductive elements arranged ina matrix-like array, the array comprising a longitudinal succession ofsets of vertical elements interleaved with a longitudinal succession ofsets of horizontal elements, alternate sets of vertical elements beingaligned longitudinally and adjacent sets being displaced from each otherin a horizontal direction, and alternate sets of horizontal elementslikewise being aligned longitudinally and adjacent sets thereof beingdisplaced from each other in a vertical direction, whereby there isformed a plurality of parallel continuously extending roughly helicalconductive paths through the matrix-like array, an electron source, andmeans for directing electrons from said source into a beam for passagein a longitudinal direction through said array in energy couplingrelation with the helical paths. D

2. In a device which utilizes the interaction between an electromagneticwave and an electron beam for amplifying the wave, an electron source,means for directing electrons from said source along an extended path,and a slow wave circuit for propagating an electromagnetic wave incoupling relation with said beam comprising a longitudinal succession ofcontiguous layers of wire-like elements, each layer including aplurality of such elements extending in a direction parallel to eachother and substantially perpendicular to and in contacting relation witheach of the elements of adjacent layers forming a plurality of parallelextending helix-like conductive paths.

3. In a device which utilizes the interaction between an electromagneticwave and an electron beam for amplifying the wave, an electron source,means for directing electrons from said source along an extended path,and a slow wave circuit for propagating an electromagnetic wave incoupling relation with said beam comprising a longitudinal succession ofcontiguous layers of wire-like elements, each layer including aplurality of such elements extending in a direction substantiallyparallel to each other and at a substantial angle and in contactingrelation with each of the elements of adjacent layers, and the elementsof alternate layers being parallel but olfset from each other in atransverse direction for forming a plurality of substantially helicalpaths through the circuit.

4. In a device which utilizes the interaction between an electromagneticwave and an electron beam for'amplifying the wave, an electron source,means for directing electrons from said source along an extended path,and a slow wave circuit for propagating an electromagnetic wave incoupling relation with said beam comprising a longitudinal succession ofcontiguous layers of wire-like elements, each layer including aplurality of such elements extending in a direction parallel to eachother and substantially perpendicular to and in contacting relation witheach of the elements of adjacent layers, and the parallel extendingelements of alternate layers being offset from each other in atransverse direction for forming a plurality of substantially helicalpaths through the circuit.

5. An interaction circuit for propagating an electromagnetic wave in alongitudinal direction comprising a plurality of contiguous sectionsforming an iterative pattern, each section of which includes fourcontiguous layers of transversely extending wire-like elements, all ofthe elements in respective layers being in contacting relation with eachof the elements in adjacent layers, the elements of the first and thirdlayers extending parallel to each other in a predetermined direction andthe elements of the second and fourth layers extending parallel to eachother and at a substantial angle with the elements of the first andthird layers, the elements of the first layer being offset from theelements of the third layer in a direction perpendicular to saidpredetermined direction, and the elements of the second layer beingoffset from the elements of the fourth layer in said predetermineddirection.

6. The combination of elements set forth in claim 5 wherein the elementsof the first and third layers are substantially perpendicular to theelements of the'second and fourth layers.

7. In a device which utilizes the interaction between an electromagneticwave and an electron beam for amplifying the wave, an electron source,means for directing electrons from said source along an extended path,and slow wave circuit means for propagating an electromagnetic wave incoupling relation with said beam comprising a plurality of straightwire-like elements contigously arranged and interleaved to form aplurality of parallel extending helix-like conductive paths, the pitchesof adjacent helix-like paths being opposite in sense. I

8. In a device which utilizes the interaction between an electromagneticwave and an electron beam for amplifying the wave, an electron source,means for directing electrons from said source along an extended path,and means for forming a plurality of parallel contiguously extendingconductive paths for propagating electromagnetic wave energy in couplingrelation with said beam, each of which describes a helix-like patternwhose end projection is a parallelogram, one side of which is common tothe parallel extending path adjacent thereto.

9. In a device which utilizes the interaction between an electromagneticwave and an electron beam for amplifying the wave, an electron source,means for directing electrons from said source along an extended path,and a slow Wave circuit for propagating an electromagnetic wave incoupling relation With said beam comprising means forming a plurality ofparallel contiguously extending helixlike conductive paths, the pitchesof adjacent helix-like paths being opposite in sense with a portion ofsuccessive turns thereof being common to adjacent ones of said parallelextending paths.

10. In a device which utilizes the interaction between anelectromagnetic wave and an electron beam for amplifying the wave, anelectron source, means for directing electrons from said source along anextended path, and slow wave circuit means for propagating anelectromagnetic wave in coupling relation with said beam comprising aplurality of helix-like conductive elements, the axis of each beingparallel to but displaced from that of the others, the varioushelix-like conductive elements extending contiguous to each other andbeing joined at points along their lengths to form a unitary structure,each of the helix-like elements of the structure being identical inconfiguration but the pitches of adjacent elements being opposite insense.

11. An interaction circuit for propagating an electromagnetic wavecomprising a plurality of sections forming an iterative pattern, eachsection of which includes four layers of transversely extendingwire-like elements, all of the elements in respective layers being incontacting relation with each of the elements in adjacent layers,elements in alternate layers being parallel to one another and elementsin contiguous layers being substantially perpendicular to one another,and elements of the first layer being interposed with elements of thethird layer and elements of the second layer being interposed withelements of the fourth layer.

12. In a device which utilizes the interaction between anelectromagnetic Wave and an electrom beam for amplifying the wave, anelectron source, means for directing electrons from said source along anextended path and means for forming a plurality of parallel extendingconductive paths for propagating electromagnetic wave energy in couplingrelation with said beam, said last-mentioned means comprising asuccession of contiguous sections forming an iterative pattern, eachsection of which includes four contiguous layers of transverselyextending wire-like elements, all of the elements in respective layersbeing in contacting relation with each of the elements in adjacentlayers, the elements of the first and third layers extending parallel toeach other in a predetermined direction and the elements of the secondand fourth layers extending parallel to each other and at a substantialangle with the elements of the first and third layers, the elements ofthe first layer being offset from the elements of the third layer in adirection perpendicular to said predetermined direction, and theelements of the second layer being ofiset from the elements of thefourth layer in said predetermined direction whereby said elementsdescribe a helix-like pattern whose end projection is a parallelogram.

13. In a device which utilizes the interaction between anelectromagnetic wave and an electron beam for amplifying the wave, anelectron source, means for directing electrons from said source along anextended path, and a slow wave circuit means for propagating anelectromagnetic wave in coupling relation with said beam, saidlastmentioned means including a succession of contiguous sectionsforming an iterative pattern, each section of which includes fourcontiguous layers of transversely extending wire-like elements, all ofthe elements in respective layers being in contacting relation with eachof the elements in adjacent layers, the elements in alternate layersbeing parallel to each other and elements of adjacent contacting layersbeing substantially perpendicular to each other, elements of the firstlayer being interposed with elements of the third layer and elements ofthe second layer being interposed with elements of the fourth layerwhereby there is formed a plurality of parallel extending roughlyhelical conductive paths, the axis of each being parallel to butdisplaced from that of the others and the pitches of adjacent helix-likeelements being opposite in sense.

14. A slow wave structure for use in connection with devices fortranslating electromagnetic waves comprising a helix-derived structurefor guiding said electromagnetic wave, said structure comprising aplurality of linear conductors lying in parallel planes, said conductorsforming two groups, each of the conductors in one group being parallelto one another, the conductors of one group being in spaced quadraturerelationship to the conductors of the other group, the conductors ofeach of said groups forming a plurality of even and odd-numbered sets,the conductors of each set lying in a plane and successive conductors insaid plane being laterally displaced from one another by a predetermineddistance, the conductors of one set of one group being conductivelyinterleaved between conductors of successive sets of the other group,the conductors of the odd-positioned sets of each group lying in arespective set of planes perpendicular to the planes of said sets, theconductors of the even-positioned sets of each group lying in arespective set of planes perpendicular to the planes of said sets, thenumber of planes in a set of planes being equal to the number ofconductors in a set of conductors, adjacent planes of even andodd-positioned sets of conductors of one group of conductors beingspaced apart by a predetermined distance substantially one-half saidfirst-mentioned predetermined distance, said structure extending formany wavelengths at the frequency of operation thereof in a directionperpendicular to the plane of said conductors.

References Cited in the file of this patent UNITED STATES PATENTS2,708,236 Pierce May 10, 1955 2,746,036 Walker May 15, 1956 2,800,604Beaver July 23, 1957 2,801,361 Pierce July 30, 1957 2,806,973 McEwan etal. Sept. 17, 1957 2,812,468 Robertson Nov. 5, 1957 2,823,332 FletcherFeb. ll, I958 FOREIGN PATENTS 691,900 Great Britain May 20, 19531,119,661 France Apr. 9, 1956

